CMOS image sensors (CIS) may convert an external optical image signal into an electrical image signal. A CIS may provide a lower operational voltage and reduced power consumption in conjunction to a charge-coupled device (CCD) image sensor. A CIS may also provide high integration, and thus, may be widely used in many fields.
FIG. 1 is a circuit diagram of a unit pixel 10 of a conventional CMOS image sensor (CIS). The unit pixel 10 can be implemented in various forms. A unit pixel 10 having a 4-transistor structure is illustrated in FIG. 1. As illustrated in FIG. 1, the unit pixel 10 includes a photodiode PD for generating optical charges due to incident light, and a plurality of transistors T1-T4. A transfer transistor T1 transfers optical charges gathered in the photodiode PD to a floating diffusion region FD in response to a transmission control signal Tx. In addition, a reset transistor T2 resets the potential of the floating diffusion region FD to the level of a predetermined power supply voltage VDD in response to a reset signal Rx and discharges optical charges stored in the floating diffusion region FD.
In addition, a drive transistor T3 acts as a source follower-buffer amplifier, and a selection transistor T4 is used for addressing. The selection transistor T4 is switched in response to a selection control signal Sx and transmits an output signal in the unit pixel 10 to the outside through an output terminal OUT.
In a correlated double sampling (CDS) operation that is generally performed in a CIS, the floating diffusion region FD may temporarily store electrons integrated or accumulated in the photodiode PD. Electrical characteristics may be further deteriorated in the floating diffusion region FD than in the photodiode PD. However, in the floating diffusion region FD, the electrons may be stored only for a relatively short time of about several tens to hundreds of nanoseconds (ns). The effect of leakage current may therefore be less than in the photodiode PD.
However, in methods such as global shutter and FD well capacitor adjustment, as the duration of the electron storage in the floating diffusion region FD increases, problems may occur due to the leakage current in the floating diffusion region FD. In other words, as the time during which electrons are stored in the floating diffusion region FD increases, image quality may be deteriorated due to the leakage current in the floating diffusion region FD.